Introduction

Super tele lenses (400mm+) tend to be rather expensive being and as such often limited to professional use. However, there're a few lenses where the manufacturers were able to keep a rather moderate price level and the Sigma AF 120-400mm F4.5-5.6 DG HSM APO OS is surely among to more popular options in this class. Besides a very long max. focal length it does also feature Sigma's Optical Stabilizer (OS, ~4 f-stop potential) and that's all for as little as around 700EUR/900US$. This may still sound a little steep in absolute terms but e.g. the Canon EF 100-400mm f/4.5-5.6 USM L IS, obviously its primary competitor, costs about 1200EUR/1600US$. Regarding this amount of price difference it may be worth to spend a thought or two ... if the Sigma is capable of performing adequately.
The lens replaces the Sigma AF 80-400mm f/4.5-5.6 EX OS and AF 135-400mm f/4.5-5.6 APO and it is very related to the Sigma AF 150-500mm f/5-6.3 DG HSM OS which simply uses a different front lens group on the same base design.

The Sigma lens is not a designated EX member (Sigma's professional grade lens segment) but the build quality is on a very high level and as good as on its EX predecessor. The lens body is obviously made of metal (crinkle style) with a few plastic parts. It is a fairly big and massive lens and, typical for such lenses, it extends quite a bit when zooming towards the long end of the range - it's certainly a eye catcher especially with an attached lens hood. The zoom and focus control ring are fairly stiff but reassuring. The front element does not rotate thanks to a rear-focusing mechanism. The lens is supplied with a dedicated tripod mount which makes, of course, sense regarding its heavy weight.

As mentioned above, the Sigma relies on a HSM drive using a fast and near-silent ultrasonic motor. Contrast/Live-View AF is about average (in the true sense). The optical stabilizer (OS) has a claimed efficiency of up to 4 f-stops. This isn't unrealistic but for the sake of a higher reliability shouldn't really try to exploit this to the max. The AF may not be overly noticeable acoustically but the OS certainly is. The OS has two modes - mode 1 is meant for general purpose photography whereas mode 2 should be used for "panning".